I'm not certain, but I don't think that there is any plumage of Bridled Tern which would look so dusky below. In particular, the chin appears dark. I take Ho Fai's point about overall upperpart tones but what about late first summer plumage Sooty Tern? Harrison (Seabirds) description of this plumage would match these birds.
So, I tend towards Sooty Tern - can I urge you to submit this record (Sooty Tern is a rarity) for consideration.
Mike Leven